46 METALLURGY OF IRON AND STEEL.
Heating the blast to 800° F. resulted in a great saving of fuel; a further heating to 1400° F. made a further saving, but much less than might be expected; while an increase to 1800° F. may not be justified unless the ore is reduced with difficulty.
SEC. Ilh.—The temperature attained ~by hot blast.—The temperature of any fire may be found by dividing the sum total of heat present by the specific heat of the resulting products. We use the heat present and not the heat produced, because the production of heat from one kilogramme of coke is the same whether hot or cold air is used, but with hot air the amount present is greater by just the quantity contained in the air. The specific heat of the coke will also be greater when hot blast is used. The specific heat of gases varies with the temperature: at 0° 0. it takes 0.306 calories to heat one cubic meter of air 1° C., but at 2000° C. it takes 0.360 calories. The formulas for finding the specific heat of some ordinary gases are as follows, the temperatures being Centigrade and the results in calories:
N, CO, 0 and H=0.306+0.000027t C03=0.374+0.00037t
The specific heat of carbon above 1000° C. is 0.5, but below 1000° C. it is less, so that the total heat in 1 kg. of carbon at t° (when t is above 1000°) is approximately 0.5—120. Assuming the value of 1 kg. carbon as 2450 calories when burned to CO, as is the case at tho tuyeres of a blast furnace, the calculation for a temperature of 1000° F.=540° C. will be as follows:
1 kg. C+4.47 c.m. air=1.87 c.m. CO+3.53 c.m. N Heat in air 4.47X-320X540= 772
Heat in carbon 0.5t— 120
Heat in carbon and air 0.5t+ 652
Heat from combustion 2450
Total heat in 5.40 c.m. of products 0.5t-f3102
Heat in 1 c.m. .0926t+574.1
Therefore, t=-------------------- —2122